While the use of dissolved air flotation devices (DAFs) in water treatment is discussed in detail in this specification, the person skilled in the art will be aware that other water treatment apparatus may be used, and such other apparatus are included within the scope of this invention.
Industrial effluent is a complex mixture comprising dissolved organic and inorganic solids, microbes, colloids and other particles, of various sizes, charges and polarities. The dynamic and composite nature of industrial effluent makes it extremely difficult to treat effectively.
Different sources of industrial effluent require markedly different treatment strategies. For example, the meat industry produces effluent containing large amounts of organic materials, whereas mining processes can generate waste water containing high levels of metal ions and minerals. The composition and flow rate of effluent streams fluctuate constantly, depending on the time of day, the processes being undertaken further upstream and even weather conditions.
Clarifiers and DAF systems are common and relatively versatile methods of treating industrial effluent. These systems have been around for decades with little innovation in relation to their design and operation. DAF systems are used in the treatment of industrial effluents from a variety of processing plants, to remove suspended solids as well as other contaminants such as oil and grease from the effluent. The effluent or feed water is added at one end of a DAF system, as depicted in FIG. 1, and treated with chemicals such as coagulants, acids and bases which act to destabilize colloidal material, causing these to come out of suspension. Upon demobilization of these materials, flocculants are typically added to collect and aggregate these particles and other contaminants to make larger particles. This is done to improve the separation of the solids from the liquid phase of the waste water. Compressed air is also introduced into the effluent stream, typically in a recirculation loop, leading to the formation of dissolved air. When this stream is released at atmospheric pressure in the DAF, the bubbles formed by the air coming out of solution draw the suspended matter to the surface of the DAF system. The floated solids can then be skimmed off the surface of the tank, resulting in clarified water being generated and discharged at the other end of the DAF system.
Due to the diverse composition of industrial effluent and the complex interactions between these components, the optimum chemical dosage to treat a particular effluent stream through a DAF system is impossible to predict theoretically. This complexity is compounded by the fact that the flow rate and composition of the effluent stream changes continuously over time. Therefore, the optimum chemical dosing mixture must be determined empirically and in situ.
Following treatment using a DAF system, the clarified water is typically either recycled or discharged into the sewer as trade waste. For example, the clarified water can be reused on-site if it is of an acceptable quality, further reducing operating costs. If the clarified water is discharged as trade waste, then the industries that discharge it need to ensure that the water meets the standards set by the relevant regulatory bodies. The discharged water usually requires further treatment by a regulatory body to comply with relevant environmental standards. The cost of treating discharged water is based on the contaminants present in the waste water; the higher the contaminant loading, the more expensive it is to treat the discharged water. Hence, most sites that generate liquid effluent will attempt to treat the water and remove as much of the contaminants as possible before discharging the water to reduce the fees payable to the relevant regulatory body.
Although the water quality can be increased by, for example, increasing the volume of certain chemicals added to the water treatment process, the additional chemical cost leads to an increase in the overall cost of the treatment process. Industries with waste water treatment plants will generally overdose these chemicals so as to not risk getting fined by the relevant regulatory body for discharge non-compliance.
Typically, the chemical dosage is determined through trial and error. The chemical service provider will typically recommend a combination of chemicals to treat the waste water based on initial trials. Generally, an operator is employed to determine the best combination of these chemicals to add to the waste water treatment system to obtain acceptable solids removal; this is done through a series of tests, termed ‘jar tests’, to simulate the conditions in a DAF system.
Jar tests involve taking samples from the effluent stream and manually dosing the samples with the chemicals that are being used in the DAF system, which are typically recommended and supplied by a specialist chemicals supplier. A number of tests, in which the chemical dosages are varied, are performed and the solids removal efficacy is determined visually or on some occasions by measurement. The samples taken from the effluent stream reflect the composition of the stream at that specific point in time. The optimum chemical dosage determined by ajar test will only be applicable to a waste water sample that has similar properties to the test sample, whereas the effluent stream may have substantially altered during the time of the jar test. Jar testing is thus an inherently inaccurate and time-consuming process which is often difficult to perform, resulting in many waste water treatment plants being operated at sub-optimal conditions, increasing chemical consumption while producing low quality treated water.
A jar test is also not an accurate representation of the process within a DAF system. A jar test is unable to adequately replicate the complex mechanics, chemistry and hydrodynamics that are present during the operation of a DAF system, which fundamentally aid in the separation of the solids from the liquid.
In reality, the composition and flow rate of an effluent stream is rarely constant. The optimum dosage of chemicals to treat a given effluent stream will change constantly, and often significantly, depending on changes occurring further upstream relating to variations in the processes as well as changes to the flow rate and composition of the effluent. In a traditional DAF system, even with an experienced, highly skilled human operator to manipulate the chemical dosages, the quality of the treated water is likely to fluctuate considerably, on account of the constantly changing nature of the effluent stream. In practice, this means that a treatment regime which meets the discharge limits one day may not meet those limits on another day, which may lead to the imposition of non-compliance fines.
Some DAF systems attempt to improve their adaptability to fluctuating inputs by implementing control systems to automatically adjust some of the parameters of the system in response to changes in the incoming effluent stream. These DAF systems often use proportional-integral-derivative (PID) control loops or components thereof. PID control loops are programmed to maintain a particular variable, such as the pH or flow rate of an effluent stream, at a constant value by adjusting a parameter of the system. PID loops are effective at maintaining variables that change in a predictable way. For example, a PID controller can maintain the pH of an effluent stream at a particular value by adjusting the proportions of acid and base in the system or proportionally increase the dosage of chemicals based on the flow rate of the effluent. However, when dealing with multiple variables that are influenced by a myriad of underlying factors, such as the dosage of coagulants and flocculants and residence time in a DAF process, PID controllers are not very effective. Optimized waste water treatment systems require the simultaneous adjustment of multiple parameters and thus require a different control method.
The management of water treatment may be performed by water treatment companies that also supply the chemicals for water treatment. This may provide a conflict of interest for the water management company between minimizing the chemical dose to reduce costs for the client and maximizing the chemical dose (e.g. applying excess qualities of chemicals and/or applying unnecessary chemicals) to increase profits for the company.